This guest post by Professor Charles J. Krebs (Department of Zoology, University of British Columbia) looks back at his experiences studying population cycles across several decades, his work with Charles Elton and the team of people who made such work possible.

In 1959 I began my Ph.D. research on lemming cycles under Dr. Ian McTaggart Cowan at the University of British Columbia (UBC), Canada. To help me get started, Ian decided to convene a symposium on cycles and invited Dennis Chitty from Oxford to come to Canada to lecture about his research on cycles. Dennis worked for his D.Phil. under Charles Elton, the Director of the Bureau of Animal Population at Oxford. Dennis gave a brilliant series of lectures on cycles, at that time one of the large, unsolved problems in population ecology. I was working on the lemming cycle for my research program, and he invited me to visit Oxford over the winter of 1960-61 in the middle of my degree. At the Bureau I talked a great deal to Charles Elton about the history of cycles research, to Dennis and Helen Chitty, and to P.H. Leslie, the local statistician at the Bureau. It was a stimulating winter, interacting with Charles Elton who had started the “Snowshoe Rabbit Enquiry” in 1931 and the “Canadian Arctic Wild Life Enquiry” in 1935. Both these questionnaire enquiries were carried on by Dennis and Helen Chitty and the staff of the Bureau. Dennis became convinced from his studies at Oxford that population fluctuations in voles were not caused by food shortage or disease but rather by social processes including stress from aggressive interactions.

In 1961 Dennis obtained from Ian Cowan an offer of a teaching position at UBC and became my thesis supervisor. Dennis was interested in teaching and was not able to do so at Oxford, so he moved to UBC. Inspired by Dennis, I finished my lemming research and then began a series of studies of Microtus population dynamics at Berkeley and Indiana University, infused now with experimental field manipulations. We wished to get at the mechanisms behind cyclic dynamics by doing field manipulations, in particular to see how social interactions might help to produce vole and lemming cycles. We discovered by the mid-1960s the “fence effect”, highlighting the role of dispersal in Microtus population dynamics. By 1970 I received an offer to return to UBC as an Associate Professor in the Zoology Department and to continue work with Dennis. Soon after this, Rudy Boonstra applied to do a Ph.D. with Dennis and thus started another set of critical field studies on grassland populations of Microtus.

Meanwhile we started research on small rodents in the southern Yukon in 1973 and the snowshoe hare cycle loomed on the horizon. The 9-10-year cycle of snowshoe hares had been highlighted by the “Snowshoe Rabbit Enquiry” started in 1931 by Charles Elton, and then spearheaded by Dennis and Helen until 1948, with the purpose of describing the pattern of these cycles across Canada and Alaska. Lloyd Keith and his students from the University of Wisconsin were carrying out hare studies in Alberta in the 1960s and we began to do experimental work in 1973 at Kluane Lake in the Yukon. Studying a 9-10-year cycle was daunting because of the long time it took to do even one experiment in the field. Stan Boutin joined our team in 1977 and by feeding hares and excluding predators over the next 20 years our research team was able to pinpoint the major role of predation mortality in generating snowshoe hare cycles.

But what was not clear was why hare reproduction began to collapse 2 years before the population peak and continued to fall during the decline and into the low phase, by which time predator numbers had collapsed and food was superabundant. Rudy Boonstra had the bright idea in the early 1990s that chronic stress might underlie these reproductive changes, and that the reproductive collapse might be a consequence of predator chases that caused stress in female hares. Advances in physiological techniques for measuring stress hormones in the 1980s and fine tuning of these methods by the 2000s allowed us to measure stress in the field.

Michael Sheriff joined Rudy and I in 2005 to start his Ph.D. on the consequences of chronic stress for snowshoe hares. Michael was able to show experimentally that chronic stress not only reduced reproduction in hares but also was inherited so that the young of stressed females were themselves stressed. The consequences of chronic stress thus moved on from generation to generation via maternal effects. For his Ph.D. research Michael Sheriff received the Elton Prize of the British Ecological Society in 2009, a fitting closing of the circle from Charles Elton’s early interest in the 9-10-year hare cycle to new research closing the gap to understanding the mechanisms behind the births and deaths that drive hare populations up and down, now summarized in our Synthesis paper.

More Info:

Krebs, C.J., Boonstra, R. and Boutin, S. (2017) Using experimentation to understand the 10‐year snowshoe hare cycle in the boreal forest of North America. Journal of Animal Ecology, 87(1): 87-100.

Sheriff, M. J., Krebs, C. J. and Boonstra, R. (2009) The sensitive hare: sublethal effects of predator stress on reproduction in snowshoe hares. Journal of Animal Ecology, 78(6): 1249-1258.